1. Field of the Invention
The present invention relates to an ink for thermal transfer recording, especially thermal transfer recording of dye-transferring type, a sheet for thermal transfer recording, and a thermal transfer recording method using the same, and a sheet set for thermal transfer recording containing the sheet for thermal transfer recording and a full color printing method using the same.
2. Background of the Invention
In dye-transferring thermal transfer recording, a thermal transfer sheet having a base film and a color material layer containing a thermally transferable dye formed on the base film is overlaid on an image-receiving sheet having a dye-receiving layer on its surface, and the thermal transfer sheet is heated to transfer the dye from the thermal transfer sheet to the image-receiving sheet and thereby to carry out recording. This recording method is applicable to full color image recording by video printers and the like. It is possible with this method to control the amount of dye transferred by controlling the quantity of the thermal energy. Thus, gradation expression is possible.
In these dye-transferring thermal transfer recording methods, the dye properties of the dye in the thermal transfer sheet and the ink composition for the thermal transfer sheet are very important, because the dye substantially influences the speed of the thermal transfer recording, the image quality, and storage stability of recorded products. Such a dye needs to satisfy the following requirements.                (1) It readily sublimates and/or thermally diffuses under the operation of the thermal recording head.        (2) It does not undergo thermal decomposition under the operation of the thermal recording head.        (3) It has a desirable hue for color reproduction.        (4) It has a large molecular extinction coefficient.        (5) It is stable against heat, light, moisture, chemicals, etc.        (6) It is easy to synthesize.        (7) It is suitable for preparation of an ink.        (8) It is safe.        
Heretofore, various dyes for thermal transfer recording have been disclosed in JP-A No. 3450/1990, No. 265792/1992, and No. 275184/1992. These references disclose as a yellow dye, a pyrazolone methine dye represented by the general formula (I) having a basic skeleton similar to the dye for use in the invention as a dye for thermal transfer recording in a sublimation method. JP-A No. 265792/1992 describes a combination of a pyrazolone methine dye and a specific dicyanomethine yellow dye. JP-A No. 275184/1992 describes a combination of a pyrazolone methine dye and a specific pyridone azo yellow dye. A quinophthalone dye is described in JPA No. 189289/1988, and an aminopyrazole azo dye is disclosed in JP-A No. 225592/1989.
However, the dyes or dye mixtures disclosed above do not perform sufficiently, and this is clear from the Comparative Examples mentioned below. Therefore, there is still a need for an ink for thermal transfer and a sheet for thermal transfer, which excel as a dye for thermal transfer recording, which exhibit a high recording density, which produce recorded products having a clear color tone, and which result in highly stable recorded products.